Emulsion polymerization processes employing polyvinyl alcohol or celluloses such as hydroxyethyl cellulose as the emulsifier, or protective colloid, yield emulsion polymer products widely used as adhesives, binders and coatings. Such products have historically been known to possess poor resistance to water, generally manifested as a loss of adhesive strength to the substrate on exposure to water or as an unattractive whitening characteristically known as "blushing". This poor water resistance property has inhibited the use of such polymers prepared in the presence of polyvinyl alcohol or celluloses as coatings coming in contact with water, for example, as interior can coatings.
To overcome this water sensitivity, crosslinking agents have been added to react with the polyvinyl alcohol of the polymer coating to render it water insoluble. However, such coatings still demonstrate the unattractive blushing on exposure to water. U.S. Pat. No. 2,843,562 attempts to overcome the water sensitivity of graft copolymers of polyvinyl alcohol and vinyl chloride by incorporating a small amount of a cross-linking monomer containing at least two olefinic unsaturations.
The inventor is aware that polymer emulsions which can deposit a polymer coating that has enhanced water resistance can be prepared by the aqueous emulsion polymerization of at least one ethylenically unsaturated monomer in a salt-free aqueous medium. The salt-free aqueous medium comprises a polyvinyl alcohol as the protective colloid and a redox system comprising hydrogen peroxide or an organic peroxy compound as the oxidant and an organic reductant. When this technology was applied to the preparation of vinyl chloride-ethylene copolymer emulsions using typical polymerization techniques including polyvinyl alcohol levels of 3 to 10 wt%, stable copolymer emulsions were not readily obtained. Another difficulty encountered is related to the solubility of the ethylene in that the ethylene pressure may rise uncontrollably during the polymerization reaction.
U.S. Pat. No. 3,399,157 teaches improving the stability of ethylene/vinyl chloride latexes by adding only a portion of the desired amount of emulsifying agent to the reaction mixture prior to the initiation of polymerization and adding a second portion to the reaction mixture after the polymerization is completed.
U.S. Pat. No. 3,501,440 discloses in Example 5 and reference Examples 5-6 the use of polyvinyl alcohol as the protective colloid in the copolymerization of ethylene with vinyl chloride at a reaction temperature between the critical temperature of ethylene and 60.degree. C. and at an ethylene pressure which is maintained substantially constant by withdrawing excess ethylene out of the reaction vessel during the polymerization.
U.S. Pat. No. 3,642,740 discloses that vinyl chloride homo- and copolymers can be prepared in aqueous emulsion using as the emulsifier system an alkali metal salt of a sulfated C.sub.8 -C.sub.18 fatty alcohol, a tallow fatty alcohol or an epoxidized unsaturated fatty acid oil, and a complex organic phosphate ester or salt derivative. Example 1 shows that the emulsion used in preparing the molding paste resin product involves the preparation of a polymer seed latex.
U.S. Pat. No. 3,689,447 discloses that heat resistant copolymers of ethylene and vinyl chloride can be prepared by the use of a seed latex in the aqueous composition for emulsion polymerization, together with heat-activated initiation at between about 50.degree. and 85.degree. C. by water-soluble persulfates or peroxydiphosphates. The seed latex can be prepared by the emulsion polymerization of any polymerizable ethylenically unsaturated compound. The seed latex can be prepared beforehand in a separate vessel and a desired aliquot can then be introduced into the aqueous composition. Alternatively, the seed latex can be made in situ in all or part of the aqueous composition before the reactor is pressurized with ethylene.
U.S. Pat. No. 3,725,367 describes a process for the preparation of polymers and copolymers having a vinyl base which comprises dispersing into the organic vinyl based monomeric system a seeding latex containing an excess amount of organo-soluble catalyst such that the seeding latex forms the dispersed phase and the organic monomeric system the continuous phase.
U.S. Pat. No. 3,736,303 discloses a latex composition comprising a copolymer of vinylidene chloride, an ethylenically unsaturated sulfur acid having sulfur in a valent state of 6, optionally an ethylenically unsaturated carboxylic acid and another ethylenically unsaturated monomer prepared using a seed latex, an ascorbic acid-hydrogen peroxide redox system, and ionic buffers and surfactants.
U.S. Pat. No. 3,875,130 discloses the preparation of homo- and copolymers of vinyl chloride in which the polymerization of the monomer composition is carried out in the presence of a seeding product prepared by the polymerization in emulsion or fine suspension.
U.S. Pat. No. 4,150,210 discloses a one-step process for the emulsion polymerization of vinyl chloride and, optionally, comonomers using a water-soluble initiator or initiator system and a mixed emulsifier of (1) a C.sub.12 -C.sub.18 straight chain alkyl or alkenyl surfactant; (2) a C.sub.14 -C.sub.20 straight chain alkyl or alkenyl alcohol; and (3) a C.sub.5 -C.sub.8 straight alkyl chain sulfosuccinate emulsifier. The examples show the use of a hydrogen peroxide-ascorbic acid redox system.
U.S. Pat. No. 4,189,415 discloses aqueous vinyl chloride-vinyl acetate-ethylene copolymer dispersions containing only polyvinyl alcohol as the protective colloid. All of the polyvinyl alcohol or only part of it can be introduced at the beginning, the ethylene pressure applied is kept constant and the polymerization temperature is 10.degree.-85.degree. C., preferably 20.degree.-50.degree. C.
U.S. Pat. No. 4,331,577 discloses a method for preparing ethylene-containing copolymer emulsions by the selective addition of the monomers mixture to the reactor in response to pressure variation and the maintenance of a monomer unsaturated condition in the reactor.